Laser gyroscopes (gyros) use a ring laser in which two counterpropagating laser beams are supported in the same laser path. The difference in frequency between the two counterpropagating laser beams is a measure of angular velocity of the laser about its central or sensing axis. At low angular velocity about the sensing axis, the frequencies of the two counterpropagating beams lock together. To use the laser as an angular rate sensor for low angular velocity, it is customary to dither the ring laser by angularly oscillating the laser about the sensing axis relative to the supporting frame or case.
Dithering causes vibrations which, if transmitted to the case, can interfere with the other instruments or gyros within a navigational system. By using an isolation arrangement for each gyro, dither vibration is not transmitted to the other instruments. An example of one isolation system which reduces the transmitted vibrational energy of an oscillating gyro may be found in U.S. Pat. No. 4,115,004, by Hutchings, et al., which issued on Sept. 19, 1978, and which is assigned to the common assignee of this invention.
The isolation system shown within the aforementioned Hutchings, et al., patent utilizes a dual-spring system, see FIG. 1, which mounts a counterweight for isolating the oscillatory motion of the gyro. This spring system includes a first set of springs mounted between the frame or case and the gyro and a second set of springs mounted between the case and the counterweight. The dual-spring system of Hutchings, et al., works well under some conditions, but is not a practical system under all operating conditions (e.g., in the presence of temperature variations) due to the need to precisely match the springs of each system. The dual system can be fine-tuned in the laboratory but, as temperatures vary, the tuning is lost thereby causing an increased amount of vibrational energy to be transmitted to the case of the gyro. One reason it is difficult to tune a dual-spring system is because the gyro case must be attached precisely at the node between the two spring systems.
An Improved Laser Gyro Dither Mechanism is shown in co-pending application Ser. No. 111,154 filed Jan. 11, 1980, by F. McNair and T. M. Wirt now U.S. Pat. No. 4,309,107 which issued Jan. 5, 1982. This application is also assigned to the common assignee of the invention. The dither mechanism shown therein utilizes a three-spring system for: first, mounting a gyro to a housing or case; second, mounting the gyro to a counterweight; and, third, mounting the counterweight to the case, see FIG. 2. Through this three-spring arrangement, it is possible to isolate substantially all angular vibrational energy which would otherwise pass to the case of a laser.
The solution set forth in the aforementioned patent application of McNair/Wirt is to decouple the node from the case such that there is only one principle resonance, namely the two-mass, one-spring free resonance. The counter-balanced dither apparatus taught by the McNair/Wirt application, as shown in FIG. 3 herein, works for its intended purpose. However, it can be seen that the supporting springs for the laser body are beneath the center of gravity of that body. This configuration with the weight of the gyro above the springs, produces an undesirable pendulous action. That is, when the gyro is accelerated in a direction normal to the sensing axis, the laser can tilt like a pendulum. The tilt can be of such a degree that, during usual random vibration levels, the gyro cannot generate data.